Limit getheaders to a hard 2000.

Previously getheaders would return any number of headers up to
2000 + the distance the locator parameter was back (which is up to
~ the number of items in the locator ^ 2).

The only client I was able to find which actually handles the headers
message is BitcoinJ, and it clearly expects no more than 2000 headers
See:
http://code.google.com/p/bitcoinj/source/browse/core/src/main/java/com/google/bitcoin/core/HeadersMessage.java#35

Additionally, the wiki clearly states that getheaders will only ever
return 2000 headers: https://en.bitcoin.it/wiki/Network#Messages